Newer designs for burner controls, such as those which heat residences, typically burn natural gas or oil as fuel. Because of the ever-increasing cost of fuels, the use of standing pilots for ignition when heat is demanded is falling out of favor. In the place of such standing pilots for lighting the main burner is either some type of direct ignition device, or an intermittent pilot burner which is relit each time there is a demand for heat. The latter design has advantages since it is typically easier to light the relatively small quantity of pilot burner fuel than the larger amount of main burner fuel, even if the main burner valve is modulated to reduce the amount of fuel which flows during ignition. It has always been relatively easy to reliably light a main burner from a pilot flame.
To assure that fuel cannot flow to the main burner until there is a pilot flame, it is customary to condition opening the main burner valve on presence of a pilot flame. To further increase safety of operation, dual redundancy is introduced into the valve design controlling the main burner fuel flow. This is accomplished with a pilot valve which controls flow of fuel to both the pilot and main valves and a main valve which controls flow of fuel to the main valve only, hence its name as the "main" (burner) valve. It is particularly important that the main valve not be open unless the pilot flame is present, since flow of main burner fuel without combustion quickly accumulates a large amount of unburned fuel which can cause an explosion or asphyxiation. In this preferred design, one can see that failure of either valve in the open position, which may be either a control problem or a problem with the valve itself, does not per se result in flow of fuel to the main burner. Even if the pilot valve fails to shut completely, the substantially lesser amount of pilot burner fuel which flows compared to the main burner flow, results in substantially less risk of harm. And if the main valve fails to close, the pilot valve prevents flow of fuel to the main burner unless fuel is also flowing to the pilot burner, which will usually light promptly to establish main burner ignition and thereby avoid a dangerous situation.
A disadvantage of this system, however, is the fact that heretofore at least two relays have been required to control these two valves. Since relays are expensive this adds cost to the system. Further, since relays are components which are inherently less reliable than other of the components which make up a burner control, each additional relay is one more opportunity for the system to fail. Relays usually fail by welding of their contacts in the closed position, meaning that current flows through the contacts even after the current flow to the control winding has ceased. To avoid the dangers inherent in this situation, it has been the practice to test the condition of the relay contacts and shut down the system if a relay contact is closed when it should be open according to the state of current flow to the control winding. But such an expedient results in additional complexity and expense of the system.